Recent electric rotating machines have been reduced in size by employment of a structure of embedding a permanent magnet in a rotor, a technique of utilizing reluctance torque by giving a magnetically salient polarity to a rotor as well as magnetic torque, and the like. One of present problems to be overcome is considered to increase a rotational speed of a rotor.
However, an iron loss (the sum of hysteresis loss and eddy current loss) induced in a rotor core is increased and particularly an iron loss induced near a surface of the iron core is increased when the rotational speed is increased. Furthermore, an eddy current loss induced in a permanent magnet embedded in a rotor core is not negligible, either. When heat generation due to these losses increases a temperature of the permanent magnet, the permanent magnet is demagnetized such that induced torque is reduced. Additionally, an increase in a temperature of the rotor core also reduces magnetic permeability of the core thereby to reduce the induced torque.
Various structures for cooling the rotor have conventionally been proposed for the purpose of suppressing a temperature increase in the rotor. For example, Patent Document 1 proposes a cooling structure in which cavities are formed so as to axially extend through a rotor core and openings are formed in both presser plates of the rotor core located opposite the cavities respectively. Partition plates which are longer than an axial dimension of the rotor are mounted in the cavities and the openings respectively. In the structure, however, the cooling performance is low since cooling by air is employed.
Furthermore, Patent Document 2 proposes a cooling structure in which cooling channels are formed on inner peripheral sides of embedded permanent magnets so as to extend through the rotor core. Each cooling channel has a section formed to be convex in a direction of the rotation center. A refrigerant is adapted to be caused to flow through the cooling channels. This structure would be insufficient in the performance to cool a rotor core although effective for the cooling of permanent magnets.    Patent Document 1; JP-A-2002-78291    Patent Document 2: JP-A-2002-345188